The use of various organic acid metal salts as a reinforcing filler in golf balls is well known and has been described in numerous patents. Rubber compounds containing these salts exhibit high hardness and good rebound. One particularly useful salt is zinc dimethacrylate which is prepared from methacrylic acid. Preparation of this and other salts often provides essentially completely hydrated forms which are less effective than the salts of the present invention.
Processes are also known for preparing the salt in situ during mixing with the rubber compound; however, there are many disadvantages. For instance, processability is not good because methacrylic acid is a liquid and it is corrosive, requiring the use of special equipment. Also, it is necessary to remove water, produced as a by-product; long mixing times are required; curing times are longer and, the properties of the compounds are variable because reaction control is difficult in situ.
With respect to the patent literature, U.S. Pat. No. 4,082,288, for instance, relates to the preparation of basic zinc methacrylate by milling under agitation methacrylic acid with a suspension of zinc oxide in a liquid medium such as water or a volatile organic liquid.
U.S. Pat. No. 4,100,182 relates to a method for preparing an adjuvant for an elastomeric composition which involves mixing methacrylic acid with zinc oxide in a liquid medium in the proportions required to form basic zinc methacrylate, removing the liquid medium and finely dividing the resulting reaction product. The reference teaches that the liquid medium may be water or a volatile organic liquid such as a hydrocarbon liquid or an alkanol.
In U.S. Pat. No. 4,500,466, owned by the Assignee of record herein, a method is provided for the preparation of zinc methacrylate powder involving the reaction of zinc oxide and methacrylic acid in a liquid aliphatic hydrocarbon such as hexane. Using this method, one obtains a mixture of the anhydrous structure as well as the hydrated structure.
Despite the existence of metal salts of carboxylic acids and their widespread use in rubber polymer compounds for a variety of products including golf balls, the present invention provides a novel form of such salts as a reinforcing filler. These salts provide superior physical properties in solid golf balls provided therewith.